In the meantime several processes for the continuous fermentation of liquid substrates have become known. DE-A 33 23 205 describes a process and equipment for the continuous fermentation of a liquid substrate with simultaneous separation of the metabolic products formed in the fermentation. It is characteristic of this method that the fermentation mixture is circulated; during the circulation a thin stream of liquid flows over a membrane surface, and the circulating fermentation mixture is pressurized to the extent that the metabolic products formed are simultaneously selectively fractionated and separated from the fermentation mixture by filtration through a membrane. To maintain continuous operation a substrate supply cycle from which fresh substrate is continuously withdrawn and led into the fermentation cycle through a sterilization module is provided upstream. Metabolic products are continuously removed from the process.
This continuous process of the prior art, however, has a number of disadvantages, which are due to characteristics of construction as well as those connected with its operation.
In the process disclosed in DE-A 33 32 205, a membrane is used to remove the metabolic products produced from the fermentation mixture. Such membranes, however, tend to become blocked, which reduces the separation efficiency and requires very large membrane surface areas. In addition, special techniques and high pressure must be used to maintain the permeability of the membrane. Also, the membrane does not permit continuous removal of sediment from the fermentation equipment or continuous separation of the cell mass formed in the fermentation.
Additionally, this process of the prior art has a number of the disadvantages described above for the batchwise process. These disadvantages in particular relate to sterilization of the equipment and media and additionally the inability to adapt the culture conditions to the prevailing growth phase of the microorganism. This disadvantage results in a diminished yield of cell mass and/or catabolite formation and simultaneously poor substrate utilization, and also leads to unacceptably long culture times.